This invention relates generally to computed tomography (CT) imaging systems, and more particularly to methods and apparatus for measuring radiation dosages in post-patient collimated CT imaging systems.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
At least one known CT imaging system employs post-patient collimation, one purpose of which is to reduce slice broadening. For example, a mechanical collimator is interposed between the patient and a radiation detector matrix. The mechanical collimator is comprised of a strip of material essentially opaque to x-rays, such as lead. The strip covers a detector assembly housing a detector array, and has a slot aligned along the detector elements to collimate x-rays impinging on the detector elements.
In imaging systems without post-patient collimation, an x-ray dose delivered to a patient can be directly measured from a measurement of the x-rays reaching the detector. However, in systems employing post-patient collimation, not all of the x-ray beam actually reaches the detector during dose testing, because some of it is blocked by the post-patient collimator. As a result, patient dosage cannot be accurately determined, because an unknown percentage of the radiation dose received by a patient never reaches the detection matrix.
It would therefore be desirable if methods and apparatus were available to provide accurate dose measurement even when post-patient collimation is employed.